Well, two kinds of particles... the shiny one, and the dynamic one.
First up: ParticleSystem. Allow 4RPL to use Unity's particle system, to make coders able to make more beautiful creations! And maybe explosions! Change values such as image used, spawn rate, life time, whether it fades, distance based, starting velocity range, etc. There is a ton of customisability with a particle system, and it's a lot easier to make things look animated instead of a lot of images.
Second: Particles! Or rather, allowing 4RPL cores to be marked as a physics object, such as a Strider or a bubble from an Airsac. I can think of a couple of uses for this, such as rockets, bombs, meteors, and shrapnel(like the shards). Just having cores act like particles in PF or bubbles from the airsac opens up a lot of doors. Maybe open up the option for multiple particles to be attached to a core, so 2 for a cylinder or maybe 8 for a box, or whatever shape is desired.
I HAVE to up this comment, because Ican already see things surf on Creeper waves:)